1. Field of the Invention
The present invention relates to a method and an apparatus for forming an image, and more specifically, to a method and an apparatus for forming an image in which an intermediate transfer body is used to form an image on a print medium in order to avoid adverse effects due to the amount of ink absorbed by the print medium.
2. Description of the Related Art
At present, as a method of forming an image using an ink jet printing system, output devices of personal computers and the like are known. Such devices mainly employ a direct printing system that forms an image by ejecting ink directly on a recording medium. The direct printing system has a simple apparatus configuration. Further, relatively inexpensive relevant products are commercially available on the market. Accordingly, this system is suitable for use as a simple printer. On the other hand, disadvantageously, output image quality depends on print media (mainly paper) and exclusive print media must be used to output an image from a digital camera so that the image has a high quality. Exclusive paper for high image quality outputs is expensive, and there are only a few types of such paper. In particular, no exclusive paper is available such as thin glossy print paper suitably used for offset printing. This is partly why the ink jet printing system is not used for commercial printing.
However, the use of the ink jet printing system enables the quality of an outputted image to be improved and facilitates coloring. Moreover, inks for ink jet printing are relatively inexpensive and there are a great variety of such inks. It has thus been desirable to allow a high-quality image to also be formed on print media other than ink jet print media, using the ink jet printing system, which has the above advantages.
The quality of output images based on the ink jet printing system depends on print media because ink jet inks are fixed by absorption of the ink into the print media. Almost all the components of the ink jet ink are liquid. Accordingly, if only a little or no ink jet ink is absorbed into the print media, an enormously long time is required for drying. If the ink jet ink contains a nonvolatile liquid, it may not appropriately dry. Moreover, inappropriate printing may occur; adjacent printed ink droplets may be mixed together (bleeding) or an ink droplet having already impacted a print medium may be drawn to an ink droplet impacting the print medium later (beading). Thus, at present, it is very difficult to form images on print media into which little or no ink jet ink is absorbed, using the ink jet printing system.
In contrast, if an image is printed on a print medium with an excessively high ink permeability, a coloring material strike-through may occur resulting in insufficient coloring or the surface of the print medium may become wavy. Likewise, if a thin print medium is used, ink strike-through caused by penetration of inks or a wavy surface such as cockling may occur. In this manner, printing by the ink jet printing system tends to be significantly limited by the amount of ink absorbed into the print medium.
Further, almost all the components of the ink jet ink are moist because the ink jet printing system enables inks of only low viscosity to be ejected. The ink jet printing system includes a continuous system, a bubble jet (registered trademark) system that is an on-demand system, and a piezo system. Any of these systems enable inks of only low viscosity to be ejected. This is because inks used for the ink jet printing system must have a high fluidity in an ink jet head in order to meet ejection appropriateness. In contrast, the inks must have a low fluidity on the print medium so as to prevent adjacent ink droplets from being mixed together on the print medium or prevent ink droplets from drawing each other as described above. In this manner, with the ink jet printing system, even though the inks with a high fluidity are ejected to the print medium, they must have a low fluidity on the print medium. Accordingly, inconsistent characteristics are required.
To meet these inconsistent demands on the inks, a printing method (also referred to herein as a “transfer ink jet printing method) has been proposed which comprises forming an ink image on a transfer body (also referred to as an intermediate transfer body) and transferring the ink image formed on the transfer body to a desired print medium to form the ink image on the desired print medium. With this system, inks ejected from the ink jet head are allowed to adhere to the intermediate transfer body. An ink image is thus formed which has a fluidity reduced to some degree. Subsequently, the ink image is transferred from the intermediate transfer body to the print medium.
With the transfer ink jet printing method, the rate of ink transferred from the intermediate transfer body to a print medium (also referred to herein as a “transfer rate”) is an important element. To transfer an ink image of a sufficient density to the print medium at a low transfer rate, it is necessary to increase the amount of ink ejected to the intermediate transfer body. However, an increase in the amount of ink applied increases the incidence of bleeding or beading, described above. Moreover, in terms of productivity, it is not preferable to increase the amount of moisture removed or a load on cleaning of the transfer body, besides the amount of ink applied.
Various methods have been proposed in order to improve the transfer rate. For example, Japanese Patent Application Laid-open Nos. 6-199032 (1994) and 7-133451 (1995) propose a method of providing a release layer on the transfer body which is used to release inks from the intermediate transfer body during transfer, in order to improve the transfer rate. However, if a liquid layer that is the release layer is provided on the transfer body, an ink image formed on this layer is not fixed. Consequently, the ink image may be “distorted,” or “bleeding” or “misalignment” may be caused by a transfer pressure exerted during transfer. That is, an image grade may be degraded. Moreover, the presence of the liquid layer requires the ink image to be dried so as not to dry the liquid layer. This hinders the drying of the ink image, thus affecting an increase in speed and the robustness of the image.
Further, Japanese Patent Application Laid-open No. 5-200999 (1993) discloses an apparatus that transfers an image to a print medium by allowing an ink jet print head to inject ink droplets into an intermediate transfer body, into which a solvent in the ink is absorbed to concentrate the ink. However, defects may occur in this apparatus; a long time may be required to absorb the solvent, or absorption holes formed in the intermediate transfer body to absorb the solvent may be clogged up. Moreover, the absorbed ink solvent may remain on the intermediate transfer body to make the image nonuniform between its parts with the ink solvent and its parts without the ink solvent. Further, it is necessary to provide means for removing the absorbed ink solvent. Accordingly, a large amount of energy is consumed in recovering the transfer body to its initial state, including the absorption of the solvent.
Thus, regardless of whether the method (direct printing) of forming an image by allowing the ink jet head to eject inks directly to a print medium or the transfer ink jet printing method is used, there remain problems to be solved in terms of the degree of freedom for available print media.
As is apparent from the above description, for printing with the ink jet printing method, a system using an intermediate transfer body is effective in increasing the degree of freedom for print media. However, even for the system using the intermediate transfer body, there remain problems to be solved in order to transfer high-quality ink images to the print medium. In particular, to form a high-quality image on a great variety of print media including those having a rough surface or a low ink absorptivity, it is necessary to transfer the inks on the intermediate transfer body to those print media at a high transfer rate.
The transfer rate is affected by the surface roughness or ink absorptivity of the print medium and thus greatly varies with the types of print media. For example, if the surface roughness of the print medium is larger than the thickness of ink formed on the intermediate transfer body, a convex portion and a concave portion of the print medium have different ink contact areas. Further, the higher the concavity of the concave portion, the smaller its contact area. In other words, the transfer rate decreases with increasing roughness of the surface of the print medium. Further, transfer conditions such as the transfer pressure must be varied depending on the ink absorptivity. Accordingly, a high transfer rate cannot be achieved unless the transfer is carried out under optimum transfer conditions. However, it is difficult to control the transfer conditions in accordance with the ink absorptivity. Therefore, it is difficult to achieve a high transfer rate when taking the ink absorptivity into account.